7Ah isn't much but most of your loads are small. Presumably you'll charge it from the outboard. Your big draw item is the bilge pump. Find out how much current it draws & divide that number by 7 to see how long the battery will power it. If the bilge pump draws 2A, the battery will power it for ~3.5 hours (probably a bit less, as that 7Ah rating is for a 20 hour rate).

One issue is that AGM batteries require a different charging regimen from a standard wet-cell. If you over charge them, they'll die very quickly. You should check what voltage the OB charges at. Most OBs I know are unregulated, which probably isn't what you want.

Surely, you've got some kind of equivalent? Some big-box buying club or company known for selling things relabelled as their own?

The $22 for a 7AH AGM is quite comparable to the price in USD for the same thing here. Batteries of all kinds here often have a 2:1 difference in price for the exact same battery from different vendors. Unless you've got, what? Stiff tariffs or excise taxes on car batteries?

Another niche is to try contacting a local amateur radio club. There's often someone in the local utility company who gives them two-year-old deep cycle batteries FOR FREE, or dirt cheap, because the backup power batteries have to be changed every second year even if they've not been used. That's not new, but often is in very good condition.